I had the same problem, so if anyone else stumbles across this problem (doubtful with such an old niche game) then you just go to the autobahn on patrol, and drive to the red dot on the map (Police HQ) to start the MadCop Part 2 mission,

Win the race and then you are told they will call you for Part 3, but my "Unlock All Cars" achievement popped after winning the race.